1. Field of the Invention
The present invention relates to techniques used in a power window apparatus for detecting that an object is caught. In particular, the present invention relates to a technique, in opening/closing a door window of an automobile, for preventing an excessive squeezing force from being applied to a caught object, and for preventing the door window from being erroneously stopped or reversed.
2. Description of the Related Art
In a known technique for detecting an object, a parameter that indicates a motor load imposed on the window glass, when opened/closed by a motor drive, is measured. Then, the measured parameter (motor load torque) is compared with a preset reference median. When the parameter is substantially deviated from the reference median, the motor is stopped or reversed based on the determination that an object is caught (see, for example, Japanese Unexamined Patent Application Publication No. 11-62380 (corresponding to U.S. Pat. No. 6,150,784)). According to this technique, the reference median is set within the entire range where an object can be detected, that is, the range between the fully-closed position and the fully-open position of the window glass, which is included in the entire moving range of the window glass.
In another known technique for detecting that an object is caught in a door window, an external force applied to the window glass is continuously monitored. Then, when a detected external force exceeds a threshold, a driving motor is stopped or reversed based on the determination that an object is caught (see, for example, Japanese Unexamined Patent Application Publication No. 11-131909). A first threshold and a second threshold that is smaller than the first threshold are used to determine whether an object is caught.
In Japanese Unexamined Patent Application Publication No. 11-62380, the reference median is a motor load torque required for a movement of the window glass when no object is caught. Practically, the motor load torque is made up of, for example, the weight of the window glass and the friction between the window glass and a sash, and varies depending on the position in the entire moving range of the window glass. Although the reference medians exist all over the moving range of the window glass, one given position in the moving range is associated with only one reference median.
In Japanese Unexamined Patent Application Publication No. 11-131909, the first threshold and the second threshold are set for detecting that an object is caught when a door of a hardtop vehicle is closed or opened. Therefore, one given position in the moving range of the window glass is associated with two thresholds. In Japanese Unexamined Patent Application Publication No. 11-62380, a value obtained by adding a certain level of an allowable reference value to the reference median is used to determine whether an object is caught. When a motor load imposed on the window glass exceeds this value, it is determined that an object is caught. This value corresponds to the thresholds described in Japanese Unexamined Patent Application Publication No. 11-131909. The allowable reference value is a value determined by converting a maximum allowable force, which can be applied to a caught object, into the motor load torque, or by making some corrections to this value. The maximum allowable force that can be applied is interpreted as an allowable squeezing force that can be applied to a caught object.
In a vehicle, especially an automobile, each window glass, particularly of front seats, normally has an inclined side in front and a horizontal side at the back. An object may be caught either at the inclined side or the horizontal side. When an object is caught at the horizontal side, an external force applied to the object can be detected as a motor load imposed on the window glass. On the other hand, when an object is caught at the inclined side, a less force, compared to the external force applied to the object, is detected as a motor load imposed on the window glass, due to the moment of force (the detail will be described in Description of the Embodiments).
Therefore, in the power window apparatus where only one threshold is provided for one given position in the moving range of the window glass, as disclosed in the above patent documents, even if an appropriate threshold, which is appropriate for detecting an object caught at the horizontal side, is set, an external force exceeding an allowable value may be applied to an object caught at the inclined side of the window. On the other hand, when a lower threshold appropriate for detecting an object caught at the inclined side is set, a minor incident occurring at the horizontal side leads to the determination that an object is caught. As a result, the motor for driving the window glass develops a tendency to malfunction and is, for example, stopped and reversed erroneously.